Problem:

You are in a house with three floors, each containing a single room with a single bulb controlled by one of three switches in the basement. You can only walk upstairs once. How can you identify which switch controls which bulb?

Scroll to the bottom of the page for the answer

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Solution:

Turn on the first switch and leave it on long enough for the bulb to get hot; turn it off, turn on the second switch, go upstairs. One bulb will be hot (first switch), one will be on (second switch), and one will be off (third switch).